Differential amplifiers are used to amplify a differential analog input signals and produce differential output signals. A differential amplifier may be configured to function as a comparator, transmitter, various amplifiers (operational, sense, etc.), an up-converter mixer, and as a gain buffer stage, and the like.
Conventional common source differential amplifier circuits rely on tuning the value of a resistor to control either the range of the voltage gain or increments of adjustment for the gain (i.e., step size). A resistor at the load of the common source differential amplifier circuit or a source degeneration resistor coupled between the sources of input transistors is tuned to increase or decrease the resistance, thereby changing the voltage gain of the common source differential amplifier circuit. In the context of the following description, the transistors are metal-oxide-semiconductor field-effect transistors (MOSFETs).
A tunable resistor is constructed as an array of switches and polysilicon resistors. The switches control the resistance by enabling one or more of the resistors in the array. However, the tunable resistor has several disadvantages. In silicon, the layout of the tunable resistor consumes a large area, especially when a resistor having a small resistance is fabricated using polysilicon because the width and length of the resistor structure are increased to construct a resistor having smaller resistance. The tunable resistor typically occupies an area that is ten times larger than the input transistors. A larger area results in greater parasitic capacitances and poor high frequency performance.
The performance characteristics of the tunable resistor do not track the characteristics of the MOSFETs for variations in fabrication process, operating voltage, and operating temperature because the structure of a resistor, as fabricated in silicon, differs from a MOSFET. Therefore process corner performance of the common source differential amplifier circuit suffers due to differing variations in the process, voltage, and temperature (PVT) performance between the MOSFETs and tunable resistor(s). For example, resistors constructed of polysilicon that are included in the tunable resistor do not necessarily respond to variations in the fabrication process, variations in voltage levels (e.g., high and low power supply voltages and signal levels), and variations in temperature during operation in the same manner as MOSFETs that are constructed of polysilicon and additional materials. Specifically, in one example, the resistance of the tunable resistor may be unchanged for variations in temperature while the switching speed of the MOSFETs increases as the temperature is reduced. Failure of the tunable resistors to track the MOSFETs across PVT variations detrimentally affects linear voltage control. Consequently, the voltage gain range is reduced and the step size is inconsistent.
Therefore, it is desirable to construct a common source differential amplifier with programmable voltage gain range and step control that is relatively consistent across PVT variations.